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Summary Information Concerning Indebtedness (Parenthetical) (Detail) - USD ($)
Dec. 31, 2017
May 01, 2017
Dec. 31, 2016
Debt Instrument [Line Items]      
Outstanding Principal $ 784,640,000   $ 750,188,000
Unsecured Credit Facility      
Debt Instrument [Line Items]      
Secured credit facility borrowing capacity 300,000,000 $ 300,000,000  
Outstanding Principal [1] $ 104,005,000    
Secured Credit Facility      
Debt Instrument [Line Items]      
Secured credit facility borrowing capacity     312,500,000
Outstanding Principal [2]     $ 150,000,000
[1] The unsecured credit facility total capacity is $300,000, of which $104,005 was outstanding as of December 31, 2017.
[2] The secured credit facility total capacity was $312,500, of which $150,000 was outstanding as of December 31, 2016.